In U.S. Pat. No. 4,383,847, processes are disclosed for making suspension fertilizers wherein a liquid waste from the production of elemental phosphorus is incorporated in fertilizer mixtures. In this case the liquid waste is bleedoff water from phosphorus condensing systems at phosphorus furnaces, and the waste contains elemental phosphorus.
In the production of wet-process phosphoric acid, a liquid waste is obtained which is a solution of phosphoric and fluosilicic acids. This is called pond water. The phosphoric acid has value as a nutrient in making fertilizers; this acid is used to make orthophosphate suspension fertilizers and solid ammonium phosphate fertilizers. Fluorides prevent the formation of large crystals of ammonium phosphate when orthophosphate suspension fertilizers are produced. Therefore the quality of the suspension fertilizer is improved by the presence of fluorides. Suspension fertilizers can be prepared by dissolving monoammonium phosphate and diammonium phosphate in water. The quality of the suspension fertilizer made by this method is improved by incorporating fluorides in the mixture. However, pond water is not suitable for use in making fertilizers for the following reasons.
1. The concentration of phosphoric acid in pond water is too low for recovery in either suspension fertilizer or solid ammonium phosphates.
2. Fluorine is present in pond water as fluosilicic acid instead of fluoride.
The present invention discloses a process for the production of wet-process phosphoric acid wherein a liquid waste is obtained which contains fluorides instead of fluosilicic acid. Also, the concentration of nutrient phosphorus in the waste is about 10 times greater than it is in pond water. The concentrated waste can be used to make fertilizers and the fluorides will improve the quality of suspension fertilizers.
The present invention is not limited to the recovery of fluorine compounds in the waste gases at wet-process phosphoric acid plants. The fluorine compounds in other waste gases can be recovered by the processes disclosed herein. Nevertheless, technology for the abatement of air pollution at wet-process phosphoric acid plants is emphasized to explain the disclosed processes.
Wet-process phosphoric acid is produced by digesting finely ground phosphate ore with sulfuric acid forming a slurry. The slurry is comprised of phosphoric acid, material insoluble in sulfuric acid, and calcium sulfate--a byproduct of the acid digestion. The slurry is filtered to separate the liquid and solid phases. The filtrate is dilute phosphoric acid which contains about 32 percent P.sub.2 O.sub.5 and is called filter-grade acid. Filter cake is a mixture of calcium sulfate and material insoluble in sulfuric acid, called byproduct gypsum. Filter-grade acid is generally concentrated by vacuum evaporation to about 54 percent P.sub.2 O.sub.5, and this acid is called merchant-grade wet-process phosphoric acid, commonly used to make fertilizers.
Phosphate ores are mineral apatites which contain both phosphorus and fluorine. The ores digested to make wet-process phosphoric acid are beneficiated and the apatite content is in the range of 80 to 90 percent. The basic mineral in phosphate ores is fluorapatite--Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 --and it has a theoretical F:P.sub.2 O.sub.5 weight ratio of 0.089. However, fluorapatite is modified in most phosphate ores whereby carbonate and fluorine are substituted for phosphate, and some calcium is replaced by other metals. The F:P.sub.2 O.sub.5 ratio is generally higher than 0.089. Fluorine present in excess of that needed for a F:P.sub.2 O.sub.5 ratio of 0.089 is more readily volatilized than the fluorine associated with P.sub.2 O.sub.5 in the fluorapatite molecule. The degree of fluorapatite modification affects the quantity of fluorine volatilized when phosphate ores are processed.
The phosphate ores mined in Florida have a F:P.sub.2 O.sub.5 weight ratio in the range of 1.1 to 1.4. Most of the wet-process phosphoric acid produced in the U.S. is made from this ore. The fluorine distribution data given in table 1 may be considered typical when wet-process phosphoric acid is produced by the dihydrate method with atmospheric pressure digestion.
TABLE 1 ______________________________________ Distribution of Flourine when Wet-Process Phosphoric Acid is Produced from Florida Phosphate Ore Percent of total F Source of flourine in phosphate ore ______________________________________ Reactor vapors 6 Byproduct gypsum 24 Evaporator vapors 41 Merchant-grade phosphoric acid 29 Total 100 ______________________________________
The gases discharging from the digester and filter are called reactor vapors and, as shown in table 1, 6 percent of the fluorine is volatilized in these vapors. The fluorine volatilizes into the surrounding air. A large volume of air must be collected by ventilating equipment to prevent fluorine from escaping and causing both air pollution and occupational health problems. The collected air has a low concentration of fluorine, but air pollution regulations are based on quantities emitted. About 99.8 percent of the fluorine in the collected air must be removed to meet air pollution emission standards.
About 24 percent of the fluorine remains in the byproduct gypsum. The quantity of byproduct gypsum made is four to five times the quantity of P.sub.2 O.sub.5 produced as acid. Practical uses or disposal methods have not been developed, and large piles of the material accumulate as solid waste.
Vapors from the evaporator contain about 41 percent of the fluorine in the phosphate ore. The fluorine from this source is prevented from being discharged in the air by condensation and water scrubbing wherein a 20 to 25 percent fluosilicic acid solution is recovered. Fluosilicic acid may be sold as a byproduct to fluoridate potable water, and several processes are known for making cryolite and aluminum fluoride from the fluosilicic acid. Cryolite and aluminum fluoride are used in the aluminum industry.
About 29 percent of the fluorine remains in the merchant-grade wet-process phosphoric acid. Part of the fluorine is volatilized when the phosphoric acid is neutralized to make ammonium phosphate fertilizers--monoammonium phosphate and dicalcium phosphate. Fluorine emitted when ammonium phosphates are produced can be recovered by the processes disclosed in the present invention.
The 6 percent of fluorine in the reactor vapors is considered in the present application. The fluorine compounds in the collected air are removed by scrubbing with water, and a typical arrangement of equipment is as follows.
1. Venturi scrubber to remove particulates.
2. Water sprays in gas duct downstream from the venturi scrubber.
3. Gross-flow scrubber.
4. Entrainment collector.
Water used in the scrubber system comes from ponds located on the pile of byproduct gypsum. The water is recirculated from the ponds to the scrubber system. Fluorine and phosphorus compounds collect in the pond water, resulting in the formation of a dilute solution of a mixture of fluosilicic and phosphoric acids. The pond water contains 2,000 to 8,000 ppm of F, present as fluosilicic acid, and about the same range of concentrations of P.sub.2 O.sub.5 as phosphoric acid. The pH of pond water is in the range of 1 to 3.
Fumes from the digester and filter contain P.sub.2 O.sub.5 as entrained particles of phosphoric acid and as particles of unreacted phosphate ore entrained in the air as dust. The entrained particles are removed when the air is treated in the fluorine scrubber system. Equipment washouts are another source of P.sub.2 O.sub.5 in the pond water. Washout water is added to pond water to avoid aqueous pollution problems. A favorable water balance can be maintained by evaporating water vapor in the scrubber and by evaporation from the pond. But the water balance may be upset by accumulation of water in the pond from rainfall. When it is necessary to discharge excess pond water, double liming of the effluent is necessary to remove fluorine and P.sub.2 O.sub.5 to meet regulations for water pollution abatement. Double liming of large quantities of water is costly.
The water balance in the scrubber system may be upset by the use of water in grinding phosphate ore. Wet grinding consumes less energy than dry grinding, and less dust is emitted in the air when the ore is ground wet. Larger quantities of water could be evaporated at the scrubber and from the ponds by increasing the pond water temperature, but air pollution standards are not met when the scrubber water temperature exceeds about 95.degree. F. Furthermore, little control can be exercised over the pond water temperature but the temperature is controlled by the weather. In the summertime it is difficult to keep the pond water temperature below 95.degree. F.
Scrubber water is recirculated to the ponds for cooling. Fluorine is emitted because of the vapor pressure of fluorine over fluosilicic acid solution in the ponds. Furthermore, fluorine may be generated in the pond by the reaction between collected particles of phosphate ore and phosphoric acid represented by the following reaction. EQU Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 +14H.sub.3 PO.sub.4 +10H.sub.2 O=10CaH.sub.4 P.sub.2 O.sub.8 .multidot.H.sub.2 O+2HF
The hydrofluoric acid formed is a volatile compound and it may be discharged in the air at ponds. Hydrofluoric acid may react with silica to form volatile silicon tetrafluoride. The reaction between silica and hydrofluoric acid is represented by the following equation. EQU SiO.sub.2 +4HF=SiF.sub.4 +2H.sub.2 O
The quantity of fluorine emitted from the cooling pond is difficult to measure accurately, but various investigators have reported rates in the range of 0.4 to 20 pounds F per acre of pond surface per day. It is generally recognized that under certain atmospheric conditions, fluorine emissions from the pond can be an environmental hazard and solutions to this environmental problem were not known previously.
Phosphate ores contain sodium and potassium compounds. Phosphate dust is collected in pond water, and some sodium and potassium compounds will be constituents of the scrubber water. These alkali metals combine with fluosilicic acid to form insoluble sodium and potassium fluosilicates. These salts precipitate as tenacious scales on scrubber packing, entrainment separators, and in pumps causing operating delays for removal. Absorption tower packing is sometimes avoided because of the problem of scale deposits, whereas more effective fluorine removal could be realized if the scrubbing system could be operated with efficient tower packing.